1. Field of the Invention
The present invention relates to a switching regulator which can improve operation efficiency during a light load.
2. Description of the Prior Art
In recent years there have been demands for electronic devices to save power in view of environmental concerns. Such demands are especially strong for battery-driven electronic devices. To advance power saving, it is important in general to reduce the power consumption of an electronic device and reduce unnecessary power consumption thereof by enhancing the efficiency of a power supply circuit. To realize the latter, a high-efficiency power supply circuit used for small-size electronic device has been developed. For example, a non-isolated switching regulator incorporating an inductor has been widely used.
There are mainly two controlling methods for such a switching regulator. One is a PWM (Pulse Width Modulation) mode and the other is a VFM (Variable Frequency Modulation) mode. In PWM mode output voltage is controlled to be constant by changing a duty ratio of an output signal (pulse wave) with a certain frequency in accordance with a change in a load or input voltage. In VFM mode output voltage is controlled to be constant by changing a frequency (cycle) of an output signal in accordance with a change in a load or input voltage. The VFM mode is also referred to as PFM (Pulse Frequency Modulation) mode.
The power consumption of a switching regulator increases in proportion to a switching frequency, that is, the number of times at which a switching transistor is turned on and off. In the PWM mode, a power loss by a periodic on-off control of a switching transistor degrades the efficiency of the switching regulator during a light load. Meanwhile, in the VFM mode the frequency of the switching regulator varies according to a load so that the switching frequency can be lowered during a light load. Accordingly, the efficiency of the switching regulator is higher in the VFM mode than in the PWM mode during a light load. Thus, during a heavy load, the switching regulator is placed in the PWM mode with merits taken into consideration while it is placed in the VFM mode during a light load.
With occurrence of reverse current in the inductor, however, the switching regulator cannot lower the switching frequency to a desired value. In view of this, in the VFM mode the switching elements of the switching regulator are disconnected to prevent a reverse flow of current in the inductor. Japanese Patent No. 4031507, for example, discloses such a switching regulator which detects an indication of occurrence of reverse current and disconnects a second switching element and a ground terminal to prevent the reverse current from flowing into the inductor.
However, in the above switching regulator reverse current may still occur in the inductor depending on a condition of input or output voltage. Thus, the switching regulator cannot decrease the switching frequency as much as it aims to. In the worst case it becomes unable to operate in the VFM mode, decreasing the efficiency thereof during a light load.